Modern turbojets are fitted with a thrust reversal device that enables a fraction of the exhaust stream leaving the turbojet to be directed towards the front of the turbojet (generally in a direction that makes an angle of about 45° relative to the forward direction of the aircraft) in order to assist the slowing down action of brakes while the aircraft is landing. Various reversal devices are in existence, and in particular devices using doors and devices using grids.
There also exist propeller engines that incorporate a thrust reversal device arranged to reverse propeller pitch.
The cockpit of an aircraft having such an engine is fitted with a throttle control device that enables the pilot to control both the rate at which fuel is admitted into the combustion chamber of the engine, and also the thrust reversal device. For this purpose, the control device comprises a mount having pivotally mounted thereon both a code wheel and a main lever or throttle lever, together with a secondary lever or thrust reversal lever, which levers are arranged to turn the code wheel. The code wheel is associated with sensors connected to an engine control unit. Each lever is movable between a rest position and a maximum actuation position. The thrust reversal lever is hinged to the throttle lever. The assembly comprising both levers is connected to the code wheel via a complex coupling system including an intermediate wheel on the pivot axis of the throttle lever and which is connected to the code wheel. Drive of the intermediate wheel by the levers is under the control of a device that prevents either lever from moving if the other lever is away from its rest position. That device relies on cams, toggle-action parts, or parts having strokes that present cusps, i.e. points where movement is reversed.
In the more elaborate systems, the stroke of each lever includes a transition zone in the immediate vicinity of its rest position. When a lever is in the transition zone and the other lever is moved away from its rest position, the first lever is returned to its rest position.
The coupling system occupies a considerable amount of space and is heavy, in particular because of the intermediate wheel. In addition, the forces transmitted are high and require parts of the coupling system to be dimensioned accordingly, thereby adding to the weight and the volume of the coupling system, particularly since any element in the coupling system between either of the levers and the code wheel must be duplicated in order to ensure the redundancy that is essential for the safety of the device.